The present invention relates to a photomask used in a semiconductor fabricating apparatus, and more particularly to a method of and an apparatus for detecting a defect on a photomask, in which a portion of a light transmitting area is so formed as to shift the optical phase of illumination light.
The resolving power of a projection aligner for projecting an image of a mask pattern on an object can be improved by shifting the phase of light passing through a photomask. In order to shift the phase of light passing through the photomask, various methods have hitherto been used. For example, a thin transparent film having a thickness corresponding to the wavelength of illumination light which is used in the projection aligner is formed at a desired position on a mask substrate, as described in Japanese patent applications JP-A-58-173,744 and JP-A-57-62,052, or a predetermined surface area of a mask substrate is etched to a predetermined depth as described in a Japanese patent application JP-A-62-189,468. Further, in the example shown in a Japanese patent application JP-A-62-067,514, auxiliary patterns provided with a thin film capable of reversing the phase of illumination light are added to a mask pattern to improve the resolution of an image of the mask pattern.
In a conventional apparatus for detecting a defect on a photomask, a photomask to be inspected is illuminated by ordinary illumination means, and it is checked whether or not the light and darkness distribution on that image of a mask pattern which is formed of light passing through the photomask agrees with the light and darkness distribution on an image which has a predetermined light-transmitting area and a predetermined light-shielding area. For example, a method of comparing the light and darkness distribution on the image formed of the transmitted light from the photomask with the light and darkness distribution obtained from design data which is recorded a magnetic tape is discussed on pages 138 to 144 of SPIE, Vol. 633, Optical Microlithography V (1986).
As can be seen from the above, the conventional apparatus for detecting a defect on a photomask pays no attention to a defect in a phase-shifting mask, and cannot detect a transparent or semitransparent defect such as a transparent or semitransparent foreign substance attached to an ordinary photomask.
The term "defect" used herein includes a defect in a thin film for introducing the optical phase shifting, a defect in a chromium film for forming a light shielding area, the remainder of etching, and others.
As mentioned above, the conventional apparatus for detecting a defect on a photomask cannot detect a defect in a transparent film which is formed in a light transmitting area to act as a phase shifter.